1. Field of the Invention
This invention pertains to a continuous process for the preparation of organic isocyanates.
2. Description of the Prior Art
The manufacture of isocyanates from primary amines and phosgene has been disclosed. Depending on the nature of the amines, the reaction is carried out either in the gas phase or in the liquid phase, either batchwise or continuously (W. Siefken, Liebigs Ann. 562, 75 (1949).
The conventional processes suffer from numerous disadvantages. If low temperatures are used for the initial phosgenation (cold phosgenation), the liberation of large amounts of phosgene during heating the reaction mixture to the final phosgenation temperature (hot phosgenation temperature) presents a problem which is difficult to deal with and which is further aggravated by the high toxicity of phosgene. Low temperature processes also suffer from a further disadvantage, namely that the rate of reaction is relatively low, so that large reaction volumes have to be handled. In the two-stage processes, the end product and the carbamoyl chloride formed as an intermediate in the first stage can react with some of the starting amine to form substituted ureas and polyureas or other undesirable products.
In a different continuous process for the preparation of isocyanates, a portion of the reaction solution is recycled at elevated pressure and elevated temperature. As disclosed in Federal Republic of Germany application No. 1,037,444 (U.S. Pat. No. 2,822,373), the phosgene solution here is mixed with an amine solution before entering the reaction vessel. Here, a certain turbulence must be generated at the point where the two solutions meet, however, the flow tube can easily become clogged, so that turbulence is no longer possible.
Federal Republic of Germany Pat. No. 1,175,666 (United Kingdom Pat. No. 827,376) discloses a process for the continuous preparation of isocyanates at elevated pressure in a reaction tube whereby the starting components are introduced into the reaction tube at a pressure exceeding 3 bar, are then reacted with increasing temperature, and the reaction mixture is drawn off through a relief valve at the end of the reaction tube to obtain the isocyanates. Since the recycled phosgene has not been subjected to hydrogen chloride purification, salts are formed and deposits build up on the mixing nozzle.
In order to improve this process, Federal Republic of Germany application No. 1,593,412 (United Kingdom No. 1,146,656 and United Kingdom No. 1,141,910) disclose the decomposition of the carbamoyl chloride into isocyanate and hydrogen chloride and the removal of the hydrogen chloride and excess phosgene from the reaction mixture in a distillation column at temperatures from 120.degree. to 180.degree. C. and at a pressure of from 10 to 50 bar, with the phosgene being refluxed.
Federal Republic of Germany published application No. 19 56 777 (U.S. Pat. No. 3,781,320 and U.S. Pat. No. 3,887,167) disclose a mixing device comprised of a chamber and a coaxially positioned rotor equipped with blades, the ends of which are positioned close to the interchamber wall, for the preparation of organic isocyanates in a one-step process. According to this publication, it is not necessary to remove hydrogen chloride from the reaction mixture since its presence does not adversely affect the process.
According to the disclosure of U.S. Pat. No. 3,829,458, organic isocyanates are manufactured continuously from primary organic amines and phosgene in an inert organic solvent in one or more packed reaction vessels, preferably with recycling of the reaction mixture, under conditions of what is called transition flow. Using this relatively simple process isocyanates can be manufactured with a high volume-to-time yield. However, in this process the introduction of the amines into the phosgene-containing reaction solution again presents certain difficulties, since with some amines optimum intermixing of the starting components can only be achieved with difficulty. If, on the other hand, the reactants are not intermixed adequately, amine hydrochlorides and ureas are formed as by-products; these in part deposit on the packing and can lead to blockage of the packed columns.
A series of known preparation methods has resulted in significant technological successes, however, all these methods have the disadvantage that excess amounts of polymers and other undesirable by-products are formed. The formation of such by-products can significantly reduce the isocyanate yield or the quality of the product.
Good results are also obtained with the process described in Federal Republic of Germany published application No. 26 24 285 (U.S. Pat. No. 4,128,569). Here the phosgene is mixed into the reaction solution, which itself is cycled in a loop and the resulting reaction mixture and the amines or amine solution is fed into the mixing and reaction zone in such a way that in this zone an energy dissipation density of from 5 to 1000 kJ is produced per m.sup.3 recycled reaction mixture plus added amine solution. However, the desired phosgenation quality also cannot be achieved with this process, especially with respect to the formation of by-products.
The objective of the invention at hand was to improve the mixing processes for the continuous preparation of isocyanates, especially the process of Federal Republic of Germany published application No. 26 24 285 and to completely, or at least partially, eliminate existing defects.